Method and apparatus for improved radiation detection

ABSTRACT

In a method for measuring detected radiation, an analog data signal is converted to a digital data signal having aperiodic data pulses varying with intensity of the analog data signal. A time signal indicative of data intervals is produced. The data pulses are counted. A data count is stored in a start location and a corresponding time value is stored in a start location each time a data pulse occurs until a measured data interval starts. After a next data interval is detected, the data count is stored in an end location and a corresponding time value is stored in an end location when the next data pulse occurs. An average intensity of the detected radiation for the measured data interval is determined from the stored data counts and time values. A CT scanner ( 10 ) for measuring detected radiation includes a channel circuit ( 56 ), a storage circuit ( 60 ), a control circuit ( 58 ), and a processor ( 62 ).

The invention relates to the diagnostic imaging arts. It findsparticular application in conjunction with defining measurement periodsfor data intervals in CT scanners and will be described with particularreference thereto. However, it is to be appreciated that the inventionis also amenable to other applications.

Analog/digital (A/D) conversion in a CT scanner utilizes an integratingcurrent to frequency converter (IFC). The IFC is a current-controlledoscillator. The current produced by a detector associated with the CTscanner varies the frequency of the current-controlled oscillator.During a data interval (which is defined by the angular position of arotating gantry or, more precisely, an arc segment), the IFC pulses arecounted, and the time from the first pulse to the last pulse is measuredwith high precision. The actual measurement is calculated by taking theratio of the COUNTS to the TIME. The precision of the measurement ishigh since it is determined by the precision of the TIME count which isproduced by counting the pulses from a high frequency oscillator.

In a “delta data” mode of operation, the counting of COUNTS and TIMEpulses starts with the last IFC pulse of the preceding data interval andends with the last IFC pulse of the measured data interval. By allowingthe measurement period to extend into the preceding data interval, allthe current from the radiation detector is utilized, thus insuring highquantum signal to noise ratio. The “delta data” technique does, however,advance (or skew) the measurement period from its physical arc segment(i.e., data interval). With a large number of COUNT pulses in the datainterval, this shift is minimal. If 100 COUNT pulses are counted, theskewing is nominally 0.5%. However, for low signal levels, this skew canbe significant. If only one pulse is generated per data interval, theskewing is nominally 50% but can be up to 100%. This data skewing maycause objectionable image artifacts.

A standard ratiometric type A/D conversion (without delta data) requiresthat at least two COUNT pulses be produced per data interval. Whenemploying delta data this requirement is reduced to one COUNT pulse perdata interval. In order to insure that the minimum pulse rate ismaintained, an offset dc current is injected into the front end. Thecounts resulting from this offset current are subsequently subtractedout before taking the ratio of COUNT to TIME. However, the shot noiseassociated with this offset current increases the input noise of the A/Dconversion thus reducing the overall dynamic range of the system.

There is, therefore, a need to improve the accuracy of previous deltadata modes by reducing (or eliminating), on the average, the skewing ofthe measurement period with respect to the measured data interval. Thereis also a desire to further reduce the required offset current in orderto minimize noise and improve the dynamic range of the system.

In one embodiment of the invention, a CT scanner includes a means forrotating a radiation source around an examination region, a means forgenerating an analog data signal that varies with an intensity ofradiation traversing the examination region, a means for converting theanalog data signal to a digital data signal including aperiodic pulsesvarying in frequency with the intensity of the radiation traversing theexamination region as the radiation source rotates about the examinationregion, a means for producing a time signal indicative of dataintervals, and a means for determining average radiation intensity ineach data interval by counting the pulses of the digital data signalstarting with a digital data signal pulse occurring in a preceding datainterval and continuing to a digital data signal pulse occurring in asucceeding data interval.

In another embodiment, the invention provides a method of measuring anintensity of detected radiation in a CT scanner. A radiation source isrotated around an examination region. An analog data signal that varieswith an intensity of radiation traversing the examination region isgenerated. The analog data signal is converted to a digital data signalincluding aperiodic pulses varying in frequency with the intensity ofthe radiation traversing the examination region as the radiation sourcerotates about the examination region. A time signal indicative of dataintervals is produced. Average radiation intensity in each data intervalis determined by counting the pulses of the digital data signal startingwith a digital data signal pulse occurring in a preceding data intervaland continuing to a digital data signal pulse occurring in a succeedingdata interval.

In still another embodiment of the invention, an apparatus for measuringan intensity of a detected radiation in a CT scanner includes a channelcircuit which generates time-based digital information from an analogdata signal for a measured data interval, the time-based digitalinformation including at least one component of the analog data signalfrom a preceding data interval and a succeeding data interval, a storagecircuit which stores the time-based digital information, a controlcircuit which determines when to store the time-based digitalinformation, and a processor which determines an average intensity ofthe detected radiation for the measured data interval from the storedtime-based digital information.

One advantage of the invention is the measurement period for a measureddata interval is, on the average, centered on the data interval, thusproducing an average measurement skew of zero.

Still another advantage is, under conditions of high attenuation, themeasurement period is significantly longer than the data interval thusproducing more integrated signal, reducing quantum noise, and increasingthe system dynamic range.

Yet another advantage is the increase in measurement period as the inputsignal decreases produces an adaptive filtering effect in the analogdomain that can potentially improve image quality by reducing noise moreeffectively than by subsequently filtering in the digital domain.

Still yet another advantage is, in various embodiments, offset currentcan be reduced to a point where less than one pulse occurs per datainterval. This reduces shot noise associated with the offset current anddecreases the effects of quantization noise and 1/f noise. The resultingoverall noise reduction improves image quality and extends the dynamicrange of the system.

Other advantages will become apparent to those of ordinary skill in theart upon reading and understanding the following detailed description.

The drawings are for purposes of illustrating exemplary embodiments ofthe invention and are not to be construed as limiting the invention tosuch embodiments. It is understood that the invention may take form invarious components and arrangement of components and in various stepsand arrangement of steps beyond those provided in the drawings andassociated description. Within the drawings, like reference numeralsdenote like elements.

FIG. 1 is a block diagram of an embodiment of a CT scanner incorporatingthe invention.

FIG. 2 is a block diagram of an embodiment of a signal processorassociated with the CT scanner of FIG. 1.

FIG. 3 is a block diagram of an embodiment of a storage circuitassociated with the signal processor of FIG. 2.

FIG. 4 is a timing diagram associated with an embodiment of theinvention in which the measurement period for a data interval extendsinto adjacent preceding and succeeding data intervals.

FIG. 5 is a timing diagram associated with another embodiment of theinvention in which the measurement period for a data interval may extendinto either of two preceding and either of two succeeding dataintervals.

With reference to FIG. 1, a CT scanner 10 includes a stationary gantry12 a rotating gantry 14, an imaging region 16, a radiation source 20, acollimator and shutter assembly 22, a subject support 30, a headrestraint 32, a plurality of radiation detectors 40 or 42, an encoder44, a signal processor 46, a reconstruction processor 48, a volume imagememory 50, a video processor 52, and a display device 54.

The stationary gantry 12 and rotating gantry 14 define the imagingregion 16. The rotating gantry 14 is supported by the stationary gantry12 for rotation about the examination region 16. The radiation source 20(e.g., x-ray tube) is arranged on the rotating gantry 14 for rotationtherewith. The radiation source 20 produces a beam of penetratingradiation that spans and passes through the examination region 16 as therotating gantry 14 is rotated by an external motor (not illustrated)about a longitudinal axis of the examination region 16. The collimatorand shutter assembly 22 forms the beam of penetrating radiation into afan, cone, or wedge shape and selectively gates the beam on and off.Alternately, the radiation beam is gated on and off electronically atthe radiation source 20. The patient support 30, such as a radiolucentcouch or the like, suspends or otherwise holds a subject being examinedor imaged at least partially within the examination region 16 such thatthe beam of radiation defines a volume through the region of interest ofthe subject. The head restraint 32 restricts the mobility of thesubject's head.

In a third generation CT scanner, an arc or a 2-dimensional array ofradiation detectors 40 is mounted peripherally across from the radiationsource 20 on the rotating gantry 14. In a fourth generation CT scanner,one or more stationary rings of radiation detectors 42 are mountedaround the stationary gantry 12. Regardless of the configuration, theradiation detectors 40, 42 are arranged to receive the radiation emittedfrom the radiation source 20 after it has traversed the imaging region16.

The radiation detectors 40, 42 convert the detected radiation intoanalog data signals. That is, each radiation detector 40, 42 produces ananalog data signal that is proportional to an intensity of receivedradiation.

The signal processor 46 receives the analog data signals from theradiation detectors 40, 42. The signal processor 46 optionally performsfiltering and other operations (e.g., generation of time-based digitalinformation and calculation of average radiation intensity per datainterval) before passing the data to a reconstruction processor 48 thatreconstructs volume image representations of the subject for storage ina volume image memory 50. A video processor 52 under operator controlretrieves and formats selected portions of the data for display on adisplay device 54, printing on a printer, etc. as a slice image,3-dimensional rendering, or the like.

During each orbit of the rotating gantry 14, the encoder 44 produces anindex signal that is transmitted to the signal processor 46 to associatethe position or angular arc segments of the rotating gantry with theanalog data signals from the radiation detectors 40, 42. Each rotationof the radiation source is broken up into a succession of individualscan segments (i.e., data intervals) as the rotating gantry 14 turns ororbits the subject. In the preferred embodiment the index signal is aseries of pulses, with a predetermined amount of pulses for each datainterval. The last pulse for each data interval indicating terminationof one data interval and initiation of a next or succeeding datainterval. In alternate embodiments, devices capable of producing asimilar index signal may be used in place of the encoder 44.

The encoder 44 produces an index signal pulse at regular angularintervals, e.g. 0.1 degree. The index signal provides a timing signaldefining the beginning and end of successive data intervals.

The signal processor 46 includes a plurality of delta data channelcircuits 56 a-56 n that are each responsive to individual analog datasignals from the radiation detectors 40, 42, a delta data controlcircuit 58 that is responsive to an index signal from the encoder 44, adelta data storage circuit 60 for accumulating time-based digitalinformation corresponding to the analog data signals, a delta dataprocessor 62, and a radiation intensity storage circuit 64. The deltadata channel circuits 56 a-56 n are typically identically constructed.

With reference to FIG. 2, only one delta data channel circuit 56 a isshown in the signal processor 46 to simplify the description. The deltadata control circuit 58 develops time-based digital information in thedelta data channel circuit 56 a corresponding to the analog data signal.This type of conversion may also be referred to as a analog-to-digital(A/D) conversion. In principle, this type of conversion is preferablyaccomplished using current to frequency conversion (IFC) or voltage tofrequency conversion (VFC) techniques. At appropriate times, the deltadata control circuit 58 transfers the time-based digital datainformation from the delta data channel circuit 56 a to the delta datastorage circuit 60 and notifies the delta data processor 62 when thedata is ready to be read from the delta data storage circuit 60.

The delta data channel circuit 56 a includes a summing module 66, anoffset module 68, an IFC 70, a data pulse detector 72, a free-runningoscillator 74, a data counter 76, and a time counter 78. The delta datacontrol circuit 58 includes a data interval detector 80 and a delta datacontroller 82.

In summary, the delta data channel circuit 56 a provides an A/Dconversion of the analog data signals by integrating a current outputand producing a pulse train of a corresponding frequency. The delta datacontrol circuit 58 monitors the data interval index signal and theoutput of the IFC 70. During scanning operations, the intensity of theanalog data signal inherently varies with tissue density.

In one embodiment, the delta data control circuit 58 stores “start datacount” and “start time” for each data interval in response to the last“pulse” of the pulse data signal in the preceding data interval. In thisembodiment, the delta data control circuit 58 also stores the “end datacount” and “end time” for each data interval in response to the firstpulse of the succeeding data interval. The delta data processor 62determines the number of pulses (i.e., COUNTS) from the differencebetween the “end data count” and the “start data count” and thedifference between “end time” and “start time” (i.e., TIME) for eachdata interval. The delta data processor 62 divides the COUNTS by theTIME to generate a numeric radiation intensity value for one datainterval for one detector. Thus, the measurement period reflected by theCOUNTS and TIME for each data interval extends into both the precedingand succeeding data intervals. This is referred to as a symmetricaldelta data mode of operation. Although the measurement period extendsoutside of the measured data interval, on the average, the measurementperiods are centered on the measured data intervals. In this manner, thesampling window dynamically widens beyond one data interval with highattenuation. The longer measurement periods reduce noise and increasethe S/N ratio during high attenuation (when a higher S/N is mostimportant), thus increasing the overall dynamic range of the A/Dconversion. This technique produces a symmetrical variable filteringmethod for measuring radiation intensity during scanning operations.

In the embodiment being described, the delta data channel circuit 56 apreferably guarantees that at least one “pulse” is output from the IFC70 during each data interval. To accomplish this, the offset module 68provides an offset current to the summing module 66. The summing module66 combines the offset current with the analog data signal to produce anoffset data signal. Preferably, the current provided by the offsetmodule 68 is adjusted to a minimum level required to guarantee that theIFC 70 generates at least one pulse during each data interval. The datapulse detector 72 monitors the output of the IFC to detect pulses. Eachtime a pulse is detected, the detected event is communicated to thedelta data controller 82.

The IFC 70 provides a digital pulse train output (i.e., pulse datasignal) that varies in frequency based on the level of the offset datasignal. As such, the pulse data signal is a digital representation ofthe analog data signal. The pulse data signal is provided to the datacounter 76. The data counter 76 counts each pulse and accumulates a“data count.” In an alternate embodiment, the “data count” can be basedon voltage rather than current. In this alternate embodiment, the IFC isreplaced with a VFC.

In the embodiment being described, the oscillator 74 of the delta datachannel circuit 56 a is free-running and provides a digital pulse train(i.e., time signal) at a relatively constant high frequency to the timecounter 78. As such, the time signal is a digital representation ofelapsed time. The time counter accumulates a “time count.” Thecombination of the “data count” and the “time count” provides time-baseddigital information representative of the radiation passing through asubject during a scanning operation. In an alternate embodiment, theoscillator 74 and time counter 78 may combined a time circuit, separatefrom the delta data channel circuits 56 a-56 n, that is common to eachdelta data channel circuit.

In the embodiment being described, the data interval detector 80receives the index signal from the encoder 44 and detects the risingedge generated during movement of the rotating gantry 14. Each pulseindicates the end of one data interval and the start of the next datainterval. Each time the rising edge of a pulse is detected, the event iscommunicated to the delta data controller 82. The delta data controller82 uses the combination of events detected by the data pulse detector 72and data interval detector 80 to determine when to process the contentsof the data counter 76 and the time counter 78 with the delta dataprocessor 62 to develop the intensity value for each data interval.Since the time-based digital information developed by the signalprocessor 46 includes data from preceding and succeeding data intervalsfor a measured data interval, the delta data controller 82 and the deltadata processor 62 may process information associated with threeconsecutive data intervals at any given time. The following descriptiondiscusses how information for the three consecutive data intervals isprocessed by referencing the second, third, and fourth data intervalsrespectively. Information associated with the second data intervalactually starts during a first data interval.

The delta data controller 82 communicates a “store” signal to the datacounter 76 and time counter 78 each time a “pulse” is detected by thedata pulse detector 72. The “store” signal directs the data counter 76and time counter 78 to transfer their current values (i.e., “data count”and “time count”) to the delta data storage circuit 60. The delta datacontroller 82 also communicates address information associated with thedelta data storage circuit 60 identifying locations in the delta datastorage circuit 60 where the data counter 76 and time counter 78 are tostore their current values.

During the first data interval, the address information identifiesstorage locations for the “start data count” and the “start time” forthe second data interval. In response to the store signal and addressinformation, the data counter 76 stores its current value in the “startdata count” location for the second data interval and the time counter78 stores its current value in the “start time” location for the seconddata interval. If a subsequent “pulse” on the pulse data signal isdetected before the next index pulse is detected by the data intervaldetector 80, the “start data count” and “start time” locations for thesecond data interval are overwritten in the same manner.

When the next index pulse is detected by the data interval detector 80,the rotating gantry 14 has reached the second data interval and theaddress information in the delta data controller 82 is altered toidentify storage locations for the “start data count” and the “starttime” for the third data interval. During the second data interval, eachtime the start of a “pulse” on the pulse data signal is detected by thedata pulse detector 72, the delta data controller 82 communicates the“store” signal and associated address information to the data counter 76and time counter 78 in the same manner as describe above. However, thedata counter 76 stores its current value in the “start data count”location for the third data interval rather than overwriting the valuestored for the second data interval. Likewise, the time counter 78stores its current value in the “start time” location for the third datainterval rather than overwriting the value stored for the first datainterval. The “start data count” and “start time” locations for thethird data interval are overwritten in the same manner if a subsequent“pulse” on the pulse data signal is detected before the next index pulseis detected by the data interval detector 80.

When the next index pulse is detected by the data interval detector 80,the rotating gantry 14 has reached the third data interval and theaddress information is altered to identify storage locations for the“end data count” and “end time” for the second data interval and the“start data count” and “start time” for the fourth data interval. Whenthe start of a first “pulse” on the pulse data signal is detected duringthe third data interval, the delta data controller 82 communicates the“store” signal and associated address information to the data counter 76and time counter 78 in the same manner as described above. However, thedata counter 76 stores its current value in both the “end data count”location for the second data interval and the “start data count”location for the fourth data interval. Likewise, the time counter 78stores its current value in both the “end time” location for the seconddata interval and the “start time” location for the fourth datainterval. The “start data count,” “end data count,” “start time,” and“end time” for the second data interval are now stored in the delta datastorage circuit 60. At this point, the delta data controller 82communicate a read signal and associated address information to thedelta data processor 62. The read signal indicates that the stored“start data count,” “start time,” “end data count,” and “end time” forthe second data interval are ready to be read from the delta datastorage circuit 60. The address information identifies the “start datacount,” “start time,” “end data count,” and “end time” locations fromwhich to read the time-based digital information for the second datainterval.

The delta data processor 62 subtracts the “start data count” from the“end data count” to determine the COUNT for the second data interval andsubtracts the “start time” from the “end time” to determine the TIME forthe second data interval. These values for COUNT and TIME relate to anaverage level of intensity for the combined offset current and analogdata signals during the second data interval. The counts produced by theoffset current are subtracted from the COUNT, and the result is dividedby the TIME to determine the intensity of the detected radiation for thesecond data interval. The radiation intensity values for each detectorand each data interval are stored in the radiation intensity storagecircuit 64 awaiting reconstruction by the reconstruction processor 48.At this point, the delta data processor 62 may communicate a read signaland associated address information to the reconstruction processor 48.The read signal indicates that the stored radiation intensity value forthe second data interval is ready to be read from the radiationintensity storage circuit 64. The address information identifies thelocation from which to read the radiation intensity value for the seconddata interval.

In another embodiment, the delta data processor 62 may accumulate thelocation information and communicate it along with the read signaleither periodically or at the completion of a scanning operation. Instill another embodiment, the radiation intensity values may be mappedinto the radiation intensity storage circuit 64 in a manner such thatthe location information need not be communicated between the delta dataprocessor 62 and the reconstruction processor 48. In this embodiment,the mapping of the radiation intensity storage circuit 64 is known tothe reconstruction processor 48. Therefore, the reconstruction processor48 only needs a read or ready signal from the delta data processor 62 orsome other device indicating that either one or more radiation intensityvalues are stored or that the scanning operation is complete.

When the next index pulse is detected by the data interval detector 80,the rotating gantry 14 has reached the fourth data interval and theaddress information is altered to identify storage locations for the“end data count” and “end time” for the third data interval and the“start data count” and “start time” for a fifth data interval. When thestart of a first “pulse” on the pulse data signal is detected during thefourth data interval, the data counter 76 stores its current value inboth the “end data count” location for the third data interval and the“start data count” location for the fifth data interval in the samemanner as described above for the second/fourth data intervals duringthe third data interval. Likewise, the time counter 78 stores itscurrent value in both the “end time” location for the third datainterval and the “start time” location for the fifth data interval. Atthis point, the “start data count,” “end data count,” “start time,” and“end time” for the third data interval are now stored and the delta datacontroller 82 communicate a read signal and associated addressinformation to the delta data processor 62 indicating such in the samemanner as described above for the second data interval. The delta dataprocessor 62 calculates a radiation intensity value for the third datainterval and stores the radiation intensity value in the radiationintensity storage circuit 64 in the same manner as described above forthe second data interval.

When the next index pulse is detected by the data interval detector 80,the rotating gantry 14 has reached the fifth data interval and theaddress information is altered to identify storage locations for the“end data count” and “end time” for the fourth data interval and the“start data count” and “start time” for a sixth data interval. When thestart of a first “pulse” on the pulse data signal is detected during thefifth data interval, the data counter 76 stores its current value inboth the “end data count” location for the fourth data interval and the“start data count” location for the sixth data interval in the samemanner as described above for the second/fourth data intervals duringthe third data interval. Likewise, the time counter 78 stores itscurrent value in both the “end time” location for the fourth datainterval and the “start time” location for the sixth data interval. Atthis point, the “start data count,” “end data count,” “start time,” and“end time” for the fourth data interval are now stored and the deltadata controller 82 communicate a read signal and associated addressinformation to the delta data processor 62 indicating such in the samemanner as described above for the second data interval. The delta dataprocessor 62 calculates a radiation intensity value for the fourth datainterval and stores the radiation intensity value in the radiationintensity storage circuit 64 in the same manner as described above forthe second data interval.

The process described above for the second, third, and fourth dataintervals is repeated for each data interval during scanning operationsas the rotating gantry 14 turns.

With reference to FIG. 3, the delta data storage circuit 60 includes adata storage block 84 and a time storage block 86. For the embodiment ofthe signal processor 46 described above, the data storage block 84 andthe time storage block 86 each include four data storage locations. Morespecifically, the data storage block 84 includes data storage locationA(0) 88, data storage location A(1) 90, data storage location A(2) 92,and data storage location B 94. The time storage block 86 includes timestorage location A(0) 96, time storage location A(1) 98, time storagelocation A(2) 100, and time storage location B 102.

With respect to the process described in reference to FIG. 2, the “startdata count” for the second data interval is stored in data storagelocation A(0) 88 and the “start time” is stored in time storage locationA(0) 96. The “end data count” for the second data interval is stored indata storage location B 94 and the “end time” in time storage location B102. Similarly, the “start data count” for the third data interval isstored in data storage location A(1) 90 and the “start time” in timestorage location A(1) 98. In the embodiment being described, the deltadata processor 62 reads the “end data count” and “end time” for ameasured data interval before the delta data controller 82 stores the“end data count” and “end time” for the next data interval. Therefore,the “end data count” for the third data interval is stored in datastorage location B 94 and the “end time” in time storage location B 102.Likewise, the “start data count” for the fourth data interval is storedin data storage location A(2) 92, the “start time” in time storagelocation A(2) 100, the “end data count” in data storage location B 94,and the “end time” is in time storage location B 102.

There are many ways of implementing the delta data storage circuit 60and the associated method for storing and reading the time-based digitalinformation representing the intensity of the detected radiation duringa scanning operation. In one embodiment, the delta data storage circuit60 is comprised of four sets of data (C) and time (T) storage locations(e.g., storage registers). The storage locations depicted in FIG. 3 areidentified in these four sets as follows: CA(0) 88 and TA(0) 96, CA(1)90 and TA(1) 98, CA(2) 92 and TA(2) 100, and CB 94 and TB 102. Thecontents of the data counter 76 and time counter 78 are transferred toone or more of the four pairs of storage locations as follows. Ondetection of a “pulse” on the pulse data signal by the data pulsedetector 56, the current values in the count and time counters aretransferred to: a) CA(0) and TA(0) for data intervals DI(1), DI(4),DI(7), etc., b) CA(1) and TA(1) for data intervals DI(2), DI(5), DI(8),etc., and c) CA(2) and TA(2) for data intervals DI(3), DI(6), DI(9),etc. On detection of the first “pulse” of each data interval, thecurrent values of count and time counters are also transferred to CB andTB. This provides the time-based digital information necessary todetermine the intensity of the detected radiation during the precedingdata interval and the stored “start data count,” “end data count,”“start time,” and “end time” are read by the delta data processor 62.

In the embodiment being described, the DATA and TIME measurements fordata intervals DI(2), DI(3), DI(4), and DI(5) are calculated by thedelta data processor 62 as follows:    DATA(2) = CB − CA(0)    TIME(2) =TB − TA(0)    (DATA and TIME measurements for DI(2) are calculatedbefore   or at the end of DI(3).)    DATA(3) = CB − CA(1)    TIME(3) =TB − TA(1)    (DATA and TIME measurements for DI(3) are calculatedbefore   or at the end of DI(4).)    DATA(4) = CB − CA(2)    TIME(4) =TB − TA(2)    (DATA and TIME measurements for DI(4) are calculatedbefore   or at the end of DI(5).)    DATA(5)=CB − CA(0)    TIME(5) = TB− TA(0)    (DATA and TIME measurements for DI(5) are calculated before  or at the end of DI(6).)    The following pseudo code performs theDATA and TIME measurements described above:    Initialize flagB = 0   for n=1:N     while DI = n, upon receipt of a count pulse     transfer counters to CA((n+2)(modulo3)) and      TA((n+2)(modulo3))      if flagB == 0        transfer counters toCB and TB        set flagB = 1      end     end     DATA(n−1) =CB−CA((n)(modulo3)) and     TIME(n−1) = TB−TA((n)(modulo3))     resetflagB = 0    end

With reference to FIG. 4, a sample timing diagram showing varioussignals within the embodiment of the CT scanner 10 described aboveduring scanning operations. As shown, the timing diagram reflects signallevels during the first six data intervals (DI1-DI6) and the last datainterval (DI N) of an exemplary detector. The INDEX signal representsthe signal provided by the encoder 44 to the data interval detector 80and defines the data intervals. As shown, the constant frequency of thepulses reflects the rotating gantry 14 moving at a constant velocity.

The DATA signal (i.e., offset data signal) represents the signalprovided by the summing module 66 to the IFC 70 produced by combiningthe analog data signal from the radiation detector 40, 42 with theoffset current from the offset module 68. The IFC signal (i.e., pulsedata signal) represents the pulse train output of the IFC 70 that isprovided to the data pulse detector 72 and data counter 76.

The OSC signal represents the free-running output of the oscillator 74provided to the time counter 78. The resolution of the diagram does notpermit identification of the frequency of the OSC signal. Nevertheless,the frequency of the pulses in the OSC signal is relatively constant ata predetermined very high frequency.

The STORE A signal represents the signal from the delta data controller82 directing the data counter 76 and time counter 78 to store currentvalues in start locations within the delta data storage circuit 60. Notethat the STORE A signal is communicated each time a pulse is detected onthe IFC signal during each data interval. The STORE A signal operates inconjunction with address information provided by the delta datacontroller 82 to the delta data storage circuit 60 to store the valuesof the counters in selected “start data count” and “start time” storagelocations for measurement of the succeeding data interval.

The STORE B signal represents the signal from the delta data controller82 directing the data counter 76 and time counter 78 to store currentvalues in end locations within the delta data storage circuit 60. Notethat the STORE B signal is communicated on the first pulse detected onthe IFC signal during each data interval. The STORE B signal operates inconjunction with address information provided by the delta datacontroller 82 to the delta data storage circuit 60 to store the valuesof the counters in selected “end data count” and “end time” storagelocations for measurement of the preceding data interval.

A pair of STORE A and STORE B signals identify the boundaries of themeasurement period for a measured data interval. Note that themeasurement period for a measured data interval starts in the precedingdata interval when the last “pulse” on the DATA signal is detected andends in the succeeding data interval when the first “pulse” on the DATAsignal is detected. This can be seen by comparing the measurementperiods to the time for the data interval, e.g., MP 2 to DI 2, MP 3 toDI 3, etc.

The READ signal represents the signal from the delta data controller 82to the delta data processor 62 indicating that the “start data count,”“end data count,” “start time,” and “end time” are stored for a measureddata interval. The delta data processor 62 uses address informationprovided by the delta data controller 82 in conjunction with the READsignal to read the time-based digital information stored for the datainterval.

In another embodiment of the method for storing and reading thetime-based digital information, the offset current provided by theoffset module 68 is reduced to a point where the delta data channelcircuit 56 a guarantees a “pulse” on the pulse data signal at least onceduring every 2½ data intervals. In this embodiment, the delta datastorage circuit 60 is comprised of six pairs of data (C) and time (T)storage locations (e.g., storage registers). Four of these pairs areused to store “start data count” and “start time count” and two are usedto store “end data count” and “end time count.” The measurement periodscan extend up to two data intervals preceding and two data intervalssucceeding the measured data interval. That is, the start pulse may betwo intervals before the measured data interval and the stop pulse maybe two intervals after the measured data interval. The measurementperiod for a measured data interval starts with the last pulse precedingthe measured data interval (i.e., within the two preceding dataintervals) and terminates with the first pulse following the measureddata interval (i.e., within the two succeeding data intervals). If nopulse is produced within the two preceding data intervals, themeasurement period starts with the first pulse of the measured datainterval or, if no pulse is produced within the two succeeding dataintervals, the measurement period ends with the last pulse of themeasured data interval. Data intervals can overlap by a greater amountat high attenuation, although on the average, the measurements will becentered on the current or measured data interval.

With reference to FIG. 5, a timing diagram shows various scenarios(i.e., scenarios a through i) for measurements periods associated with ameasured data interval (n) for the embodiment having a “pulse” on thepulse data signal as least once during every 2½ data intervals. Twopreceding data intervals are identified as n−2 and n−1. Two succeedingdata intervals are identified as n+1 and n+2. If one or more data pulsesare detected and the data interval includes a start or end boundary fora measurement period, a first line 104 is identified in the scenario. Iftwo or more data pulses are detected and the data interval includes thestart and end boundary for a measurement period, a second line 106 isidentified in the scenario. If data pulses are not detected during adata interval, a blank data interval 108 is identified in the scenario.If it does not matter whether or not data pulses are detected during adata interval, a dashed line 110 is identified in the scenario. Ameasurement period with start and stop boundaries is identified bybracket 112 in each scenario.

If no data pulses are detected for three consecutive data intervals, anerror condition exists for this embodiment. Scenario i creates asituation in which two or more data pulses are required during datainterval n. Otherwise, only one pulse is required in a data intervalused as a start or end boundary for the measurement period.

In an embodiment using storage locations, the storage location areidentified in these six sets as follows CA(0) and TA(0), CA(1) andTA(1), CA(2) and TA(2), CA(3) and TA(3), CB(0) and TB(0), and CB(1) andTB(1). In general, the contents of the data counter 76 and time counter78 are transferred to one or more of the four pairs of storage locationsas follows.

On detection of a “pulse” on the pulse data signal by the data pulsedetector 72, the contents of the data counter 76 and time counter 78may, for example, be transferred to:

-   -   a) CA(0) and TA(0) for data intervals DI(3), DI(7), DI(11),        etc.,    -   b) CA(1) and TA(1) for data intervals DI(4), DI(8), DI(12),        etc.,    -   c) CA(2) and TA(2) for data intervals DI(5), DI(9), DI(13),        etc., and    -   d) CA(3) and TA(3) for data intervals DI(6), DI(10), DI(14),        etc.        The pseudo code below for the embodiment with six pairs of        data (C) and time (T) storage locations identifies additional        storage combinations.

On detection of the first pulse within a data interval, the contents ofthe data counter 76 and time counter 78 may, for example, be transferredto:

-   -   a) CB(0) and TB(0) for data intervals DI(3), DI(5), DI(7), etc.        and    -   b) CB(1) and TB(1) for data intervals DI(4), DI(6), DI(8), etc.        The pseudo code below for the embodiment with six pairs of        data (C) and time (T) storage locations identifies additional        storage combinations.

This provides the time-based digital information necessary to determinethe intensity of the detected radiation during a data interval for theembodiment being described. The stored “start data count,” “end datacount,” “start time,” and “end time” are read by the delta dataprocessor 62 at the end of the second succeeding data interval.

The following pseudo code performs the DATA and TIME measurements forthe embodiment described above with reference to FIG. 5: initializeflagB(0) = 0; flagB(1) = 1 for n = 1:N   while DI = n, upon receipt of acount pulse    transfer counters to CA((n+1)(modulo4)) and    TA((n+1)(modulo4))    if flagB(0) == 0    transfer counters to CB(0)and TB(0)    end    if flagB(1) == 0    transfer counters to CB(1) andTB(1)    end    if flagB(0) AND flagB(1) == 0      transfer counters toCA((n)(modulo4)) and       TA((n)(modulo4))    end    set flagB(0)=1   set flagB(1)=1   end   COUNT(n−2) = CB(n(modulo2)) − CA(n+2(modulo4))  TIME(n−2) = TB(n(modulo2)) − TA(n+2(modulo4))   transfer counters toCA((n+2)(modulo4)) and TA(n−2(modulo4))    and to CB(n(modulo2)) and TB(n(modulo2))   reset flagB(n(modulo2)) = 0 end

In summary, the various embodiments described above provide what may bereferred to as a symmetrical delta data mode for measuring the intensityof detected radiation for data intervals during scanning operations in aCT scanner. The symmetrical delta data mode produces a measurementperiod for a measured data interval that extends into both the precedingand succeeding data intervals. On the average, the measurement period iscentered on the measured data interval, thus producing an average skewof zero. As a result, artifacts due to data skewing are reduced fromthose of previous delta data modes. Moreover, under conditions of highattenuation, the measurement period is significantly longer than thedata interval thus producing a more integrated signal and reducingquantum noise, thereby thus increasing the dynamic range of the overall.

The increase in measurement period as the input signal decreasesproduces an adaptive filtering effect in the analog domain that canpotentially improve image quality more effectively than subsequentfiltering in the digital domain. In various alternate embodiments, theoffset current can be reduced to less than one “pulse” in the pulse datasignal per data interval. The reduction in the offset current decreasesshot noise associated with the offset current. In addition, reducing theoffset current decreases the effects of quantization noise and 1/fnoise. The resulting overall noise reduction improves image quality andcan significantly extend the system dynamic range.

While the invention is described herein in conjunction with exemplaryembodiments, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art. Accordingly,the embodiments of the invention in the preceding description areintended to be illustrative, rather than limiting, of the spirit andscope of the invention. More specifically, it is intended that theinvention embrace all alternatives, modifications, and variations of theexemplary embodiments described herein that fall within the spirit andscope of the appended claims or the equivalents thereof.

1. A CT scanner, comprising: a means for rotating a radiation sourcearound an examination region; a means for generating an analog datasignal that varies with an intensity of radiation traversing theexamination region; a means for converting the analog data signal to adigital data signal including aperiodic pulses varying in frequency withthe intensity of the radiation traversing the examination region as theradiation source rotates about the examination region; a means forproducing a time signal indicative of data intervals; a means fordetermining average radiation intensity in each data interval bycounting the pulses of the digital data signal starting with a digitaldata signal pulse occurring in a preceding data interval and continuingto a digital data signal pulse occurring in a succeeding data interval.2. The CT scanner as set forth in claim 1, the time signal producingmeans further including: a means for detecting a start of a firstmeasured data interval and a start of a next data interval.
 3. The CTscanner as set forth in claim 2, the determining means furtherincluding: a means for storing a first digital data signal pulse countin a first start data location and storing a first time signal valueassociated with the first digital data signal pulse count in a firststart time location each time a pulse occurs on the digital data signaluntil the first measured data interval starts and for storing a seconddigital data signal pulse count in an end data location and storing asecond time signal value associated with the second digital data signalpulse count in an end time location when the next pulse occurs on thedigital data signal after the start of the next data interval isdetected; wherein the determining means determines the average intensityof the detected radiation for the first measured data interval bydividing a difference between the pulse count stored in the end datalocation and the pulse count stored in the first start data location bya difference between the value stored in the end time location and thevalue stored in the first start time location.
 4. The CT scanner as setforth in claim 3 the converting means further including: a means foradding a minimized offset signal to the analog data signal so that theintensity of the analog data signal is such that at least one aperiodicpulse occurs on the digital data signal during each data interval;wherein the determining means considers the minimized offset signal whendetermining the average intensity.
 5. The CT scanner as set forth inclaim 1, the converting means further including: a means for adding aminimized offset signal to the analog data signal prior to theconverting so that the intensity of the analog data signal is such thatat least one aperiodic pulse occurs on the digital data signal every 2½data intervals.
 6. A method of measuring an intensity of detectedradiation in a CT scanner, the method comprising: a) rotating aradiation source around an examination region; b) generating an analogdata signal that varies with an intensity of radiation traversing theexamination region; c) converting the analog data signal to a digitaldata signal including aperiodic pulses varying in frequency with theintensity of the radiation traversing the examination region as theradiation source rotates about the examination region; d) producing atime signal indicative of data intervals; e) determining averageradiation intensity in each data interval by counting the pulses of thedigital data signal starting with a digital data signal pulse occurringin a preceding data interval and continuing to a digital data signalpulse occurring in a succeeding data interval.
 7. The method as setforth in claim 6 wherein step e) further includes: f) storing a firstdigital data signal pulse count in a first start data location andstoring a first time signal value in a first start time location eachtime a pulse occurs on the digital data signal until a first measureddata interval starts; g) detecting a start of the first measured datainterval and detecting a start of a next data interval; h) after thestart of the next data interval is detected, storing a second digitaldata signal pulse count in an end data location and storing a secondtime signal value in an end time location when the next pulse occurs onthe digital data signal; and i) determining an average intensity of thedetected radiation for the first measured data interval by dividing adifference between the pulse count stored in the end data location andthe pulse count stored in the first start data location by a differencebetween the value stored in the end time location and the value storedin the first start time location.
 8. The method as set forth in claim 7,further including: in step c), adding a minimized offset signal to theanalog data signal prior to the converting so that the intensity of theanalog data signal is such that at least one aperiodic pulse occurs onthe digital data signal during each data interval; and in step i),considering the minimized offset signal when determining the averageintensity.
 9. The method as set forth in claim 7, further including: instep a), adding a minimized offset signal to the analog data signalprior to the converting so that the intensity of the analog data signalis such that at least one aperiodic pulse occurs on the digital datasignal every 2½ data intervals; in step f), continuing to store thedigital data signal pulse count in the same manner until the start of asecond data interval; in step g), detecting a start of the secondmeasured data interval between the start of the first measured datainterval and the start of the next data interval; and in step i),determining the average intensity for the second measured data intervalrather than the first measured data interval and considering theminimized offset signal when determining the average intensity.
 10. Themethod as set forth in claim 6 wherein: in step c), adding a minimizedoffset signal to the analog data signal prior to the converting so thatthe intensity of the analog data signal is such that at least oneaperiodic pulse occurs on the digital data signal every 2½ dataintervals.
 11. The method as set forth in claim 10 wherein step e)further includes: f) storing a first digital data signal pulse count ina first start data location and storing a first time signal value in afirst start time location each time a pulse occurs on the digital datasignal during first and second preceding data intervals until a firstmeasured data interval starts, wherein the first preceding data intervalis adjacent to the first measured data interval and the second precedingdata interval is adjacent to the first preceding data interval.
 12. Themethod as set forth in claim 11 wherein step e) further includes: g)detecting a start of the first measured data interval and detecting astart of a first succeeding data interval adjacent to the first measureddata interval; h) after the start of the first succeeding data intervalis detected, storing a second digital data signal pulse count in a firstend data location and storing a second time signal value in a first endtime location when the next pulse occurs on the digital data signalduring the first succeeding data interval; and i) determining an averageintensity of the detected radiation for the first measured data intervalby dividing a difference between the pulse count stored in the first enddata location and the pulse count stored in the first start datalocation by a difference between the value stored in the first end timelocation and the value stored in the first start time location.
 13. Themethod as set forth in claim 11 wherein step e) further includes: g)detecting a start of the first measured data interval, detecting a startof a first succeeding data interval adjacent to the first measured datainterval, and detecting a start of a second succeeding data intervaladjacent to the first succeeding data interval; h) after the start ofthe second succeeding data interval is detected, storing a seconddigital data signal pulse count in a first end data location and storinga second time signal value in a first end time location when the nextpulse occurs on the digital data signal during the second succeedingdata interval; and i) determining an average intensity of the detectedradiation for the first measured data interval by dividing a differencebetween the pulse count stored in the first end data location and thepulse count stored in the first start data location by a differencebetween the value stored in the first end time location and the valuestored in the first start time location.
 14. The method as set forth inclaim 11 wherein step e) further includes: g) detecting a start of thefirst measured data interval and detecting a start of a first succeedingdata interval adjacent to the first measured data interval; h) when thestart of the first succeeding data interval is detected, storing asecond digital data signal pulse count in a first end data location andstoring a second time signal value in a first end time location; and i)determining an average intensity of the detected radiation for the firstmeasured data interval by dividing a difference between the pulse countstored in the first end data location and the pulse count stored in thefirst start data location by a difference between the value stored inthe first end time location and the value stored in the first start timelocation.
 15. The method as set forth in claim 14 wherein: the firstsucceeding data interval is a second measured data interval; and step e)further including: j) storing a third digital data signal pulse count ina second start data location and storing a third time signal value in asecond start time location each time a pulse occurs on the digital datasignal during first and second preceding data intervals with respect tothe second measured data interval until the second measured datainterval starts, wherein the first preceding data interval is adjacentto the second measured data interval and the second preceding datainterval is adjacent to the first preceding data interval; wherein stepe) further includes: k) detecting a start of the second measured datainterval and detecting a start of first and second succeeding dataintervals with respect to the second measured data interval, wherein thefirst succeeding data interval is adjacent to the first measured datainterval, wherein the second succeeding data interval is adjacent to thefirst succeeding data interval; l) after the start of the secondsucceeding data interval is detected, storing a fourth digital datasignal pulse count in a second end data location and storing a fourthtime signal value in a second end time location when the next pulseoccurs on the digital data signal during the second succeeding datainterval; and m) determining an average intensity of the detectedradiation for the second measured data interval by dividing a differencebetween the pulse count stored in the second end data location and thepulse count stored in the second start data location by a differencebetween the value stored in the second end time location and the valuestored in the second start time location.
 16. The method as set forth inclaim 10 wherein step e) further includes: f) detecting a start of thefirst measured data interval; g) when the start of the first measureddata interval is detected, storing a first digital data signal pulsecount in a first start data location and storing a first time signalvalue in a first start time location.
 17. The method as set forth inclaim 16 wherein step e) further includes: h) detecting a start of afirst succeeding data interval adjacent to the first measured datainterval; i) after the start of the first succeeding data interval isdetected, storing a second digital data signal pulse count in a firstend data location and storing a second time signal value in a first endtime location when the next pulse occurs on the digital data signalduring the first succeeding data interval; and j) determining an averageintensity of the detected radiation for the first measured data intervalby dividing a difference between the pulse count stored in the first enddata location and the pulse count stored in the first start datalocation by a difference between the value stored in the first end timelocation and the value stored in the first start time location.
 18. Themethod as set forth in claim 17 wherein: the first succeeding datainterval is a second measured data interval; and step e) furtherincluding: k) storing a third digital data signal pulse count in asecond start data location and storing a third time signal value in asecond start time location each time a pulse occurs on the digital datasignal during first and second preceding data intervals with respect tothe second measured data interval until the second measured datainterval starts, wherein the first preceding data interval is adjacentto the second measured data interval and the second preceding datainterval is adjacent to the first preceding data interval; wherein stepe) further includes: l) detecting a start of the second measured datainterval and detecting a start of a first succeeding data interval withrespect to the second measured data interval, wherein the firstsucceeding data interval is adjacent to the second measured datainterval; m) when the start of the first succeeding data interval isdetected, storing a fourth digital data signal pulse count in a secondend data location and storing a fourth time signal value in a second endtime location; and n) determining an average intensity of the detectedradiation for the second measured data interval by dividing a differencebetween the pulse count stored in the second end data location and thepulse count stored in the second start data location by a differencebetween the value stored in the second end time location and the valuestored in the second start time location.
 19. The method as set forth inclaim 16 wherein step e) further includes: h) detecting a start of thefirst measured data interval, detecting a start of a first succeedingdata interval adjacent to the first measured data interval, anddetecting a start of a second succeeding data interval adjacent to thefirst succeeding data interval; i) after the start of the second datainterval is detected, storing a second digital data signal pulse countin a first end data location and storing a second time signal value in afirst end time location when the next pulse occurs on the digital datasignal during the second succeeding data interval; and j) determining anaverage intensity of the detected radiation for the first measured datainterval by dividing a difference between the pulse count stored in thefirst end data location and the pulse count stored in the first startdata location by a difference between the value stored in the first endtime location and the value stored in the first start time location. 20.The method as set forth in claim 19 wherein: the first succeeding datainterval is a second measured data interval; and step e) furtherincluding: k) storing a third digital data signal pulse count in asecond start data location and storing a third time signal value in asecond start time location each time a pulse occurs on the digital datasignal during first and second preceding data intervals with respect tothe second measured data interval until the second measured datainterval starts, wherein the first preceding data interval is adjacentto the second measured data interval and the second preceding datainterval is adjacent to the first preceding data interval; wherein stepe) further includes: l) detecting a start of the second measured datainterval and detecting a start of a first succeeding data interval withrespect to the second measured data interval, wherein the firstsucceeding data interval is adjacent to the second measured datainterval; m) after the start of the first succeeding data interval isdetected, storing a fourth digital data signal pulse count in a secondend data location and storing a fourth time signal value in a second endtime location when the next pulse occurs on the digital data signalduring the first succeeding data interval; and n) determining an averageintensity of the detected radiation for the second measured datainterval by dividing a difference between the pulse count stored in thesecond end data location and the pulse count stored in the second startdata location by a difference between the value stored in the second endtime location and the value stored in the second start time location.